This invention relates to a method for manufacturing spiral-welded steel pipe using hot-rolled steel strip or plate as material.
Spiral-welded steel pipe is manufactured on a spiral-welded pipe mill where strip or plate is continuously formed into spiral pipe form and the abutting side edges thereof are welded together to close the spiral seam. Welding is commonly performed by the submerged-arc method because it provides good bead form and several other advantages.
Yet, submerged-arc welding is not without some problems. Carried out on the spiral-welded pipe mill, the speed of welding is limited (approximately 3 m/min maximum), imposing considerable restrictions on productivity. Blowholes and other internal defects resulting from the intake of air lower the quality of the welded zone.
In order to enhance the welding speed and weld quality, attempts have been made to weld the seam by the multi-wire submerged-arc method. With inside submerged-arc welding carried out simultaneously with forming, however, it has been difficult to choose an appropriate welding position and, therefore, enhance the welding speed and weld quality.
In order to increase the welding speed, inside and outside welding are usually conducted at points 7 and 8 that are situated on the upstream (or minus) side, which is opposite to the direction in which the seam 5 of a spiral pipe 2 advances, of the lowermost point 3 and the uppermost point 4 of the pipe 2 respectively, as shown in FIGS. 1 and 2. The reason why such welding positions 7 and 8 are chosen is that a weld made on the downstream (or plus) side would reach an inclined position before there is enough time for the steel melted by the welding heat to solidify. As a consequence, the molten steel would flow out entailing an imperfect bead form. In other words, the welding positions 7 and 8, which might be situated on the plus side when welding is performed at lower speed, must be shifted to the minus side as the welding speed increases. As the welding speed is increased, the distance Lcos.theta. between the inside welding position 7 and the lowermost point 3 and the distance L' cos.theta. between the outside welding position 8 and the uppermost point 4 grow larger.
However, it should be noted that inside welding is effected at a point where one side-edge of strip 1 that comes into the line tangentially to the pipe 2 meets the rearmost side-edge of the formed pipe 2. Increasing the distance L cos .theta., therefore, enlarges the gap between the two side-edges, with a resulting melt-down or other imperfect welding.
For the enhancement of productivity, it has been proposed to provide continuous high-speed tack welding by the CO.sub.2 gas shielded arc method on a spiral-welded pipe mill and then a submerged-arc welding off the line.
With this method also, the probability of blow-holes and other defects developing is strong. In addition, it is necessary to remove a large quantity of spatter and get rid of noxious gases and fumes. Although the welding speed of this method (approximately 9 m/min maximum) is higher than that of submerged-arc welding, still higher speed has become desirable in order to attain greater productivity.